The basic image-forming process of silver halide photography comprises the exposure of a silver halide photographic element to actinic radiation (for example, light or X-rays), and the manifestation of a usable image by the wet, chemical processing of the material. The fundamental steps of this processing entail, first, treatment (development) of the photographic element with one or more developing agents wherein some of the silver halide is reduced to metallic silver.
With black-and-white photographic elements, such metallic silver can serve directly as a useful image. With color photographic materials, the useful image consists of one or more images in organic dyes produced from an oxidized developing agent formed where silver halide is reduced to metallic silver. Following development, the photographic material may be further treated with a variety of solutions to achieve specific desired effects.
With black-and-white photographic elements, development usually produces a material containing both a metallic silver image in exposed areas, and (undeveloped) silver halide in unexposed areas. Silver halide is light-sensitive, and over time may be converted to metallic silver directly by the action of light, a process commonly referred to as print-out. To prevent contamination of the chemically developed metallic silver image by print-out silver, undeveloped silver halide is generally removed from the developed material through treatment with a solution containing a silver halide solvent commonly referred to as a fixing agent. The topic of fixing agents and their use in photographic processing is thoroughly discussed by G. I. P. Levenson in The Theory of the Photographic Process, Fourth Edition, T. H. James (ed.), Macmillan Publishing Co., Inc., New York, 1977, Chapter 15, and by L. F. A. Mason in Photographic Processing Chemistry, Second Edition, The Focal Press, London, 1975, Chapter VI.
In an alternative process, commonly referred to as a black-and-white reversal process, undeveloped silver halide is left in the black-and-white photographic material after development, but the metallic silver formed as a result of development is removed in a separate processing step. After the metallic (developed) silver has been removed, the photographic material is given a second exposure to actinic radiation sufficient to cause it to be convertible to metallic silver in a subsequent (second) development step. The result of this alternative process is a metallic silver image in areas of the material that were initially not exposed, giving a "reversal" of the image formed in the process in which the development is followed by fixing.
The processing solutions used to oxidize metallic (developed) silver in a photographic material are commonly referred to as bleaches. A thorough discussion of photographic bleaches is also given by Levenson and in Chapter VII of Mason in the references cited above. In addition to their use in the black-and-white reversal process just described, bleaches are also commonly used in the processing of color photographic materials to remove the metallic silver produced by development and, thereby, prevent desaturation of the color dye images.
All photographic bleaches are solutions of oxidizing agents capable of converting metallic silver to silver ions. Specifically, the oxidizing (bleaching) agents convert silver from an oxidation state of zero (silver metal) to silver in an oxidation state of +1. Simultaneously, the oxidizing agent is reduced. As discussed by Levenson in the cited reference, the oxidizing power required of the bleach may be quantified in terms of an electrochemical potential whose value depends on the concentration of free silver ions in the material being bleached. The higher the free silver ion concentration, the greater the oxidizing power required of the bleaching agent.
After chemical development, silver is present in a photographic material as metallic silver and as undeveloped silver halide. Since the solubility of each of these materials is negligible in water, the concentration of free silver ions in the material is negligible, and bleaching will begin if the material is exposed to a bleach solution containing even a weak oxidant. As the oxidation of metallic silver proceeds and free silver ion increases, the electrochemical potential needed to continue silver oxidation increases. Only bleaches containing oxidants with very high electrochemical potentials will be able to oxidize all of the metallic silver in a developed photographic material in the presence of the free silver ion products of bleaching. A table showing the electrochemical potentials of oxidants useful in photographic bleaches is given on page 448 of the Levenson reference. If the level of free silver ion is kept low, oxidants of weaker oxidizing power are useful in photographic bleaches. One way to maintain a low concentration of free silver ion in a bleaching photographic material is to include in the bleach composition chemical species that will combine with free silver ion as it is generated by the oxidation of developed (metallic) silver. Two common classes of materials used for this purpose are halides that react with free silver ions to form insoluble silver halides within the photographic materials, and silver ion complexing agents. These complexing agents are generally also silver halide solvents, and bleaching solutions containing such complexing agents may act as a fixing bath as well as a bleach. Single solutions used to both bleach and fix a developed photographic material are commonly referred to as bleach-fixing solutions or bleach-fixers. Thus there are three possible types of photographic bleaches, differing in the final disposition of the silver ions formed from the oxidation of metallic silver. Rehalogenating bleaches contain halide ion in the bleaching solution. After treatment with a rehalogenating bleach all silver in the photographic material is in the form of silver halide. This silver halide is subsequently removed from the material in a fixing step.
Bleach-fixers are bleaching solutions containing silver halide solvents. Use of a bleach-fixer eliminates a separate fixing step. All silver, both developed metallic silver and undeveloped silver halide, is removed from a processed photographic material in a bleach-fixer.
Finally, simple bleaches contain no material, for example, halides or silver ion complexing agents, which will significantly lower the concentration of free silver ions produced by the oxidation of metallic silver formed by development. These simple bleaches are sometimes referred to as direct or non-rehalogenating bleaches. Since these simple bleaches contain no silver halide solvents, they have no effect on undeveloped silver halide, so that when a simple bleach is used, developed silver in the photographic material being bleached dissolves into the bleach while undeveloped silver halide is removed into a fixing bath.
Not all oxidants are useful in all three types of bleaches, and the formulation of a useful composition for each of the three types of bleaches requires a careful balance of the electrochemical potential of the oxidant with the redox properties of all other species in the bleach solution and in the material to be bleached. For example, only the most powerful oxidants are useful in direct bleaches. Among these are soluble salts of dichromate and permanganate. As noted above, direct bleaches based on these materials are useful in a black-and-white reversal process. On the other hand, direct bleaches based on dichromate and permanganate are often too powerful to use in bleaching developed silver in color photographic materials because they can oxidize dye images, or in bleach-fixers because they are powerful enough to oxidize commonly used fixing agents. Other oxidants, such as those based on Fe(III) salts or Fe(III) complexes, are too weak to oxidize silver without additional species (like halide or a silver ion complexing agent) in the bleach to lower the concentration of free silver ion formed as bleaching proceeds. With bromide, for example, Fe(III) compounds such as ferricyanide and Fe(III)-EDTA are capable of oxidizing silver rapidly enough to be useful as bleaches in the processing of color materials but are not so powerful as to cause oxidation (loss) of color image dyes. Since most good fixing agents are more readily oxidized than color image dyes, bleach-fixing solutions are generally of lower oxidative power than either direct or rehalogenating bleaches. As a result, their use is restricted to processes designed for specific photographic materials. Process RA-4 used for color photographic paper containing emulsions with high levels of chloride does employ a bleach-fixer based on an Fe(III) complex, but no useful bleach-fixer for photographic materials containing emulsions of low chloride levels, for example, high speed color negative films, is commercially available.
Because the utility of an oxidant in a particular type of bleach depends critically on its electrochemical potential, it may not, in general, be assumed that oxidants useful in one type of bleach will also be useful in another. For example, one cannot prepare a useful simple (non-rehalogenating) bleach by taking a rehalogenating bleach based on Fe(III)--such as the KODAK FLEXICOLOR BLEACH III--and simply removing the halide.
In addition to selecting bleach components based on the need to oxidize developed silver, there is increasing concern with regard to the effects that oxidants and other bleach components may have on the environment. Powerful oxidants such as those based on Cr(VI) and Mn(VII) and weaker agents such as aminopolycarboxylic acid chelates of Fe(III) are of concern from the point of view of environmental pollution. Concerns center around the effects of the heavy metal ions themselves and around chelating agents like EDTA commonly employed with Fe(III) that may help transport heavy metals to the soil and aqueous environment. It is one purpose of the present invention to provide methods of bleaching that have minimum negative environmental effects.
On the basis of their electrochemical potential and innocuous bleaching products, peroxy compounds such as persulfates and peroxides offer attractive alternatives to heavy metal ion bleaches. Persulfate bleaching agents that produce sulfate ion as the bleaching byproduct, have low environmental impact. Although persulfates are powerful oxidants based on their electrochemical potential, it has been found that persulfate bleaches are slow to oxidize silver in developed photographic materials, and to achieve useful bleaching rates require the use of a bleach accelerating agent.
Like persulfates, hydrogen peroxide also has an electrochemical potential that suggests that it might be useful as an oxidant in photographic bleaches. The electrochemical potential of hydrogen peroxide is high enough to suggest its use in direct bleaches. In addition, the reduced form of hydrogen peroxide--the result of its oxidation of metallic silver--is water, which is excellent from an environmental perspective. In Oxygen, Elementary Forms and Hydrogen Peroxide, published by W. A. Benjamin, New York, 1965, M. Ardon teaches that persulfates can decompose to form hydrogen peroxide in aqueous solutions below pH 1. At higher pH values, at pH 2 to 6, however, persulfate does not act as a hydrogen peroxide precursor. Thus, hydrogen peroxide must be provided in other ways.
Not surprisingly, numerous attempts to use hydrogen peroxide as a photographic bleach have been made, yet no hydrogen peroxide based bleach has found its way into the photographic trade. One problem with many hydrogen peroxide bleach formulations has been stability. Another is the tendency of hydrogen peroxide based bleaches to produce vesiculation (blistering) in photographic materials and to show incomplete bleaching.
Still, the use of hydrogen peroxide in combination with various compounds has been described. For example, U.S. Pat. No. 4,301,236 (Idota et al) describes a rehalogenating bleaching composition containing a combination of hydrogen peroxide, an organic metal complex salt such as Fe(III)-EDTA or Fe(III)-HEDTA, and an unsubstituted or substituted aromatic sulfonic acid. The presence of the sulfonic acid is said to increase the shelf stability (keep stability) of the hydrogen peroxide formulation. The patent also teaches that hydrogen peroxide alone is not a useful oxidant for bleaching color photographic materials (column 2, lines 50-54). Contrary to this teaching, the present invention describes useful bleaches in which hydrogen peroxide is the only oxidant.
U.S. Pat. No. 4,277,556 (Koboshi et al) describes a photographic bleaching composition containing acidic formulations of hydrogen peroxide with lower alkyl aliphatic carboxylic acids and/or alkylidene diphosphonic acids or alkali metal salts thereof.
In addition, WO 92/01972 describes a method of processing a photographic material that includes a redox amplification dye image-forming step and a bleach step using hydrogen peroxide. Other disclosures include U.S. Pat. No. 4,454,224 and WO 92/07300 that describe alkaline hydrogen peroxide solutions, and Japanese specifications 61/250647A and 61/261739A that describe hydrogen peroxide bleaches requiring bleach accelerators.
Despite all of the work being done to develop hydrogen peroxide bleaches, there is a continuing need for methods of bleaching using commercially viable hydrogen peroxide bleaches that are stable and non-vesiculating. It is the object of this invention to provide useful methods of processing with nonrehalogenating hydrogen peroxide bleaches that are simple and effective.